Using competition and light estimates to predict diameter and height growth of naturally regenerated beech seedlings growing under changing canopy conditions

In a naturally regenerated beech (Fagus sylvatica L.) stand in northeastern France, five regeneration patches were selected with different degrees of canopy closure; the percentage of above-canopy light (PACL) ranged between 5 and 45 per cent. The canopy was removed by a storm 2 years later. In order to analyse the immediate and delayed effects of PACL on seedling diameter and height growth, the effects of the light conditions prevailing before canopy opening on seedling growth measured before and after canopy opening were tested. The effects of local density were examined using neighbourhood analysis. Four different competition indices (CIs) and neighbourhood radii ranging from 10 to 200 cm were tested. Models including PACL and CI accounted for between 56 and 64 per cent of the variation in individual seedling annual diameter or height growth. Local density had a strong negative influence on diameter growth and a much smaller influence on height growth. PACL was positively correlated with diameter and height growth before canopy opening. Delayed effects of PACL on height growth were observed just after canopy opening, but disappeared after 2 years. No delayed effects of PACL on diameter growth after canopy opening were observed.